1. Technical Field
The present invention relates to diaryl azo pigments exhibiting improved process heat stability and to printing inks containing them having improved dried ink transparency.
2. Background
Diaryl pigments are well known in the art and are particularly useful as coloring components in various types of printing inks, as well as coloring components for paints and plastics. Typical of such pigments are those prepared by coupling bis-diazotized 3,3'-dichloro-4,4'-diaminobiphenyl with an acetoacetarylide such as acetoacetanilide (also known as acetoacetylated aniline).
A particularly desirable application for such pigments is as a coloring component in solvent based printing inks containing alcohol, which are used for packaging or label printing.
A most important characteristic for such pigments would be that they produce inks that are very transparent over metal foil. In the ideal case of a completely dispersed pigment, transparency relates to the primary pigment particle size. Generally a pigment composed of smaller particles gives more transparency. However, in reality, transparency also relates to the amount of pigment dispersion achieved in the ink. Dispersability is related to forces that bind individual particles together. These forces can be simply termed "weak" which is known as "agglomeration" or "strong" which is also known as "aggregation". These forces can be depicted as either edge-to-edge or edge-to-face binding in the case of agglomeration or as face-to-face binding in the case of aggregation. Therefore transparency is strongly influenced by the particle aggregation-agglomeration phenomenon which must be considered during the development of a pigment product.
Transparency and dispersability also relate to particle size distribution. If the primary pigment particles have a broad size distribution the larger-than-average particles may give a disproportionately large decrease in the transparency than the smaller-than-average particles give toward increasing the transparency. A wider particle size distribution also decreases dispersability in the ink medium by increasing aggregation. Two pigments with the same average particle size but with different particle size distributions can thus have quite different effective transparencies.
While it is possible to provide pigments having good dried ink transparency in the laboratory, it has been the experience that a significant loss of transparency occurs when the laboratory process is scaled up to production. This is believed caused by the fact that a loss of dried ink transparency properties occurs during the drying of production pigment presscake. Drying times and temperatures may vary in production, and a narrow processing temperature window below about 90.degree. C. needs to be maintained to preserve the dried ink transparency properties of the pigment. Also, the pigments are not sufficiently heat stable at temperatures where the production pigment may be more efficiently dried (90.degree. C. or more). Other contributing factors to the loss of transparency are thought to be caused by changes in the amount of aggregation-agglomeration bonding of the pigment particles as a consequence of the presscake extrusion, drying and milling operations.
It is disclosed in UK Patent application No. 2160212A that the solution stability of printing inks containing metal salt, naphthol-type azo pigments can be improved by incorporating into such pigments minor amounts of the diazonium salt of an aromatic amine having a carboxylic acid group or sulfonic acid group, meta or para to the amino position, coupled with beta-naphthol or beta-hydroxynaphthoic acid. The incorporation may be made by blending the naphthol-type azo pigment with the coupled aromatic amine or by co-coupling during the production of the metal salt azo pigment. In addition, U.S Pat. No. 4,665,163 discloses that the heat stability of certain diaryl pigments may be improved by coupling bis-diazotized 3,3'-dichloro-4,4'-diaminobiphenyl with a mixture of two different acetoacetanilides, one of which has an amino or amido-containing substituent group on the phenyl ring. This improvement in heat stability permits the pigments to be utilized for pigmenting plastics processed at temperature in excess of 200.degree. C., without degrading or color changes. The former reference (UK) relates to a pigment of a different chemical class than the pigments of this invention. Also, neither of these disclosures relate to the production of diaryl type pigments exhibiting improved transparency in printing ink formulations.